1. Field of the Invention
The present invention relates to a liquid crystal display, and particularly to a vertical alignment (VA) liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (LCD) is one of the most widely used flat panel displays. An LCD includes two panels provided with field-generating electrodes such as pixel electrodes and a common electrode and a liquid crystal (LC) layer sandwiched there between. The LCD displays images by applying voltages to the field-generating electrodes to generate an electric field in the LC layer, which determines orientations of LC molecules in the LC layer to adjust polarization of incident light.
A commonly used LCD mode is a vertical alignment (VA) mode LCD (VA-LCD), which aligns LC molecules such that the long axes of the LC molecules are perpendicular to the panels in absence of electric field. The VA-LCD mode exhibits several advantages such as: good viewing angle performance, high contrast ratio due to its excellent black state (independent of temperature or chromatic), low operating voltages, and a cost effective fabrication process (as it is a rubbing free process).
The good viewing angle properties are obtained by creating multi-domains in the pixel design. This can be done by using mechanical protrusions, as disclosed in U.S. Pat. No. 7,295,274, or slits in the ITO electrodes, as disclosed in U.S. Pat. No. 6,424,398, or a combination of both. The slits create fringing fields which direct the switching of the LC. The slope of the protrusions has a similar effect. From their initial homeotropic orientation, which is perpendicular to the glass substrate, the specially chosen dielectrically negative LC molecules tend to reorient perpendicular to the electrical field. With protrusions or slits, the molecules tilt in a defined direction as an electrical field is applied.
The switching time of VA mode LCD is limited by the material and cell configuration. But it is also limited by what is referred to as the reverse flow effect (or backflow effect). This phenomenon occurs if a too high voltage is applied to a VA cell and inversely results in a longer switching time. This phenomenon has been described in the following references: [1] De Gennes and Prost, Physics of Liquid Crystals 2nd Ed, Oxford; Clarendon Press, (1995); [2] Chandrasekar S., Liquid Crystals, 2nd edition, Cambridge University Press, (1992); [3] Roosendaal, Dessaud, Hector, Hughes, Boer, IDRC conference proceeding, 10-3, 127-130 2006; [4] Dessaud, Roosendaal, Hector, Hughes, Boer, IDW'06 Digest, LCT7-2, 651-654, 2006; [5] Sang Soo Kim, Brian H. Berkeley, Kyeong-Hyeon Kim, and Jang Kun Song, J. Soc. Inf. Display 12, 353 (2004).
It is known that a more uniform switching could be obtained by increasing the number of domains in a display. However, the approaches in the prior art would unfavourably reduce the total aperture ratio as, because of the shape of the electrode, some areas of LC will never switch and this will reduce the total aperture ratio. Furthermore, when the pixel size gets smaller, the size of the areas of an LC cell that will never switch will remain the same. As a result the percentage of never switching area will increase, resulting in a reduction of total aperture ratio.
The VA-LCD mode is very interesting in both transmissive and reflective mode making transflective displays possible. Optical foils play an important role in the final front of screen performance of the display.
A good reflective VA-LCD can be obtained by placing a circular polarizer on top of the display and a reflector after the LC-layer. A circular polarizer can be obtained by combination of a linear polarizer and a quarter wave plate between the linear polarizer and the LC layer. In its OFF mode, the display appears black and in its ON mode, maximum transmission can be reached. FIG. 3a shows the simulated optical response of a 45 μm pixel between crossed polarizers 0°-90°. FIG. 3b shows the simulated optical response of a 45 μm pixel between crossed polarizers 45°-135° and FIG. 3c shows the simulated optical response of a 45 μm pixel between circular polarizers. Finally, FIG. 3d shows the director profile of the LC molecules in a prior art 45 μm pixel. It is clear that, circular polarizers provide the best aperture ratio in combination with a 45 μm pixel.
In transflective displays, the need to have a good working reflective area (normally black) makes the use of circular polarisation very interesting. A good match between transmission and reflective mode curves can be obtained by using the double cell gap approach.
There are several reasons to prefer linear polarizers over circular polarizers. They provide higher contract ratio, less retardation films, thinner polarizer stack, lower manufacturing cost and stronger against the deviation of the retardation film properties. Furthermore, it is commonly known that the off-axis performance of circular polarizers is lower than the off-axis performance of linear polarizers. Furthermore, in transmissive mode the quarter lambda wave plate is not necessary. By omitting the quarter lambda wave plate in a transflective display, the off-axis performance can be improved.
For very small pixel sizes and especially for squared pixels, the LC alignment can not allow the use of linear polarizers as the loss in aperture ratio is too high when a “flower type” optical response occurs. FIG. 4a-d shows simulated optical responses from top to bottom of a 100 μm pixel, a 60 μm pixel, a 45 μm pixel and a 25 μm pixel between two crossed polarizers placed at 45°-135°. As can be seen, the loss in aperture ratio increases as the pixel size decreases.
In Shibazaki-san et. al, “57.5L: Late-News Paper: MVA Mode with Improved Color-wash-out for mobile Applications”, Society for information display 2007 International symposium, SID 07 DIGEST, 1665-1668, a new MVA technology for wider viewing angle is described. A new pixel structure with an optimized ITO slit design on the Colour Filter (CF) side is described which increases the transmittance of a transmissive MVA mode with a linear polarizer. The ITO slit provides an alignment of the LC molecules in four directions (two orthogonal directions) and therefore allows the use of linear polarizers in such configuration. The disadvantages of etching the colour filter substrate are: stricter ITO etching design rules (i.e. the ITO slits and gap minimum size) than on the TFT substrate, and difficult to etch the CF substrate without damaging the colour filters.
FIG. 5a shows an embodiment of an ITO hole 52 in the electrode 50 on the CF side. FIG. 5b shows the director profile of the LC molecules in a cell with said ITO hole. FIGS. 5c-d show the optical response of such a cell with ITO hole on the CF side between circular polarizers and crossed polarizers 0°-90°, respectively. The black spots in the middle of the cells corresponds to the part of the LC layer that is not switched as an electric field is missing at the location of the ITO hole in the electrode 50 on CF side. The unswitched area of LC layer is related to the size and the shape of the ITO hole. This effect decreases the aperture ratio of the LCD.
Therefore, it is desirable to have a LCD design which can overcome some of the disadvantages of the known designs.